1. Field of the Invention
The present invention relates to a receiver for detecting a multi-frequency signal, particularly to the receiver which has an improved function in detecting the multi-frequency signal, in which detection error is remarkably reduced.
The multi-frequency signal herein used is a signal which comprises at least two elemental signals having a different frequency. A push-button signal is known as a typical example of the multi-frequency signal, which is generated in a pushphone terminal and transmitted to a telephone exchanger as a dialing signal. The push-button signal widely used contains two signals of a different audio frequency and a combination of the frequencies corresponds to a specific selection of dialing of the pushphone terminal.
The multi-frequency signal receiver is used in such as a telephone exchanger and detects a numeral or other specific function comprised in the multi-frequency signal.
2. Description of the Related Art
The system using the push-button (hereinafter abbreviated as PB) signal in Japan is similar as that developed by the Bell Laboratories and has been utilized for practical application in the United States.
The system is briefly called a 4.times.4 signal method, in which frequencies of the PB signal are grouped into two, a low frequency group and a high frequency group. Each group has four elemental frequencies, and the PB signal is formed by a combination of two elemental frequencies selected from each group.
FIG. 1 shows a table of a frequency allocation structure in the 4.times.4 signal method. As clearly seen from FIG. 1, all elemental frequencies in the 4.times.4 signal method are included in an audio-frequency band.
The most important problem in detecting the PB signal having audio frequency components exists in that the detection error percentage will not vanish because of existence of voice or noise signals.
On the other hand, when the protective measures for reducing the detection error are too excessive, it results in diminishing a normal PB signal and also causing detection error when noises are included in the circuits. Therefore, in a practical application, appropriate protective measures should be considered.
Among preventive measures for reducing detection error, a method in which a third Formant is emphasized, is considered to be the most effective way and has been widely used in Japan.
Because vowels are apt to be the cause of detection error, a spectrum structure of vowel has attracted particular attention in the 4.times.4 PB signal method. The above method of emphasizing the third Formant is one of solutions for reducing detection error due to the vowel [a:] with high efficiency.
FIG. 2 shows a typical spectrum pattern of the vowel, for example, [a:], which often becomes a cause of detection error.
With regard to the spectrum shown in FIG. 2, the third Formant, known as a frequency range of 2.5 to 3.0 KHz denoted by a reference A, is considered to be the cause of detection error. In the known existing system, the PB signal is first input to a third Formant emphasizer and subsequently to a limiter. It is known that the emphasized component works as a negative factor in recognizing the signal as a PB signal in detection and, therefore, this method works fairly effective for reducing the detection error.
FIG. 3 shows an example of a block diagram of the existing PB signal detection system utilizing the third Formant emphasizer.
In FIG. 3, an input signal is applied to a dialtone remover (DTR) 11, in which the dialtone is suppressed. The output signal from the DTR 11 is applied to a third Formant emphasizer (3F) 12, where the third Formant is emphasized. The output of the third Formant emphasizer 12 is applied to both a band elimination filter (BEF) 13 and a band elimination filter (BEF) 14, where signals of the high frequency group are cut off by BEF 13, and signals of the low frequency group is cut off by BEF 14 respectively.
The output of BEF 13 is applied to a limiter (LIM) 15 and, after the amplitude is limited, it is further applied to band pass filters (BPF) 16, 17, 18 and 19, each having a specified pass band around frequencies of, i.e., 697 Hz, 770 Hz, 852 Hz and 941 Hz respectively of the low frequency group. When the input signal comprises anyone of above elemental frequencies, it is extracted and is output to a logic circuit 24.
In the similar way, the output of BEF 14 is applied to a limiter (LIM) 20 and, after the amplitude is limited, it is further applied to band pass filters (BPF) 21, 22 and 23, each having a specified pass band around frequencies of, i.e., 1209 Hz, 1336 Hz and 1477 Hz respectively of the high frequency group. When the input signal comprises anyone of above elemental frequencies, it is extracted and is output to the logic circuit 24.
In a logic circuit 24, a combination of the two frequencies from the band pass filters of low and high groups is checked, and the logic circuit outputs a signal corresponding to the numeral or other functional key on a pushphone.
In PB signal detection system, a detection error percentage has been considered to be allowable up to the same level as the dialing error percentage. Under this condition, the existing PB signal detection system as shown in FIG. 3 can satisfy the above condition and can perform sufficient function. However, recently it becomes still more necessary to detect PB signal with detection error as few as possible.
In a high speed packet communication, for example, PB signal detection is required to be more precise. Further, a new telephone service such as an audio-response service has been under way which includes a problem of detection error.
With regard to the audio-response service, a short explanation is given herein. FIG. 4 is a schematic block diagram of the audio-response service. Assuming that one subscriber terminal 51, for instance, calls another terminal 52, and there is no one to attend the terminal 52, a telephone exchanger 50 connects the terminal 51 to an audio message manager 53, which includes an audio message storing and reproducing circuit 55, an audio guidance transmission circuit 56 and a PB signal receiver 57. The audio guidance transmission circuit 56 first sends an audio guidance message to the subscriber of terminal 51 to input the caller's and the partner's telephone numbers by push-button operation. With this guidance, PB signal receiver 57 is made active for receiving the PB signal. Thereafter, an audio message from the terminal 51 is recorded in the audio message storing and reproducing circuit 55. Later on, the audio message manager 53 calls the partner's terminal 52 and transmits the recorded audio message.
In the system used for audio-response service, voice signal and other noise signals introduced to the PB signal from the subscriber terminal 51 and/or the reproduced audio message from the audio message storing and reproducing circuit 55 will easily become the cause of detection error for the PB signal.
In order to reduce the PB signal detection error, Japanese Unexamined Patent Publication Tokkaihei-1-243690 published on Sep. 28, 1989 by O. Aiiso discloses that the input signal is subjected to a band elimination filter, which interrupts transmission of the signal in the PB signal band covering both low and high frequency groups and, if the output signal from the band elimination filter is detected to have frequency components outside the PB signal band, the input signal is regarded as a signal other than the PB signal and the input signal is not transmitted to the PB signal receiver.
Japanese Unexamined Patent Publication Tokkaihei-1-138889 by H. Takeshita published on May 31, 1989 discloses a PB signal detection circuit comprises a Discrete Fourier Transform (DFT) circuit and a threshold value determination circuit and a judging circuit, wherein the spectrum of an input signal in the low frequency band, such as lower than 1000 Hz, is analyzed by the DFT circuit and, when the number of spectrums exceeding the threshold value is larger than a predetermined number, the input signal is judged to be a voice signal. Japanese Unexamined Patent Publication Tokkaihei-1-188194 by H. Takeshita published on Jul. 27, 1989 discloses the similar PB signal detection circuit as described above except that the DFT circuit in the latter invention is provided for analyzing the signal in the high frequency band, such as between 1100 Hz and 3000 Hz.